This energy comes from the strong nuclear force. This powerful but short-range force holds the protons and neutrons tightly together. It must be very strong to overcome the natural repulsion between positively charged protons.
When you measure a nucleus, its mass is slightly less than the combined mass of its separate pieces. Chemists call this missing amount of matter the mass defect. When the nucleus first forms, this missing mass turns directly into pure energy.
The conversion follows Albert Einstein‘s famous equation, E=mc2. Because the speed of light is so large, a tiny amount of lost mass creates a huge amount of binding energy. Students often get confused by the word binding.
They think binding energy is energy currently stored inside the nucleus. Actually, it is the energy you must add from the outside to break the nucleus apart. Scientists often calculate the average binding energy per nuclear particle.
This average value tells us how stable a specific atomic nucleus really is. Elements near the middle of the periodic table have the highest binding energy per particle. This peak makes metals like iron and nickel the most stable elements in the universe.